Although this invention has broad application to the measurement of the ratio of fine granular material within a porous material, the following exemplary description will be specifically directed to the measurement of toner concentration in an electrophotographic machine. Generally, the process of electrophotographic printing includes the following series of process steps. (a) Charging a photoconductive member to a substantially uniform potential so as to sensitize its surface. (b) Exposing the charged photoconductive surface to a light image of an original document to be reproduced, thereby discharging lightstruck areas and recording an electrostatic latent image on the photoconductive member corresponding to the informational areas contained in the original document. (c) Developing the latent image by bringing a developer mixture into contact therewith, for forming a powder image on the photoconductive member. (d) Transferring the powder image to a copy sheet. (e) Permanently affixing (usually by heat) the powder image to the copy sheet.
A common type of developer mixture frequently used in electrophotographic printing machines comprises two components. The first component is carrier granules, usually on the order of 100 .mu.m to 150 .mu.m in diameter. The second component is toner particles, usually on the order of 10 .mu.m in diameter. The toner particles are heavily loaded with pigment and adhere triboelectrically to the carrier particles for being transported thereby. When the mixture passes over the electrostatic latent image on the photoconductive member there is a preferential attraction of the toner particles for the latent image. Thus, during the printing process the toner particles are constantly being depleted from the developer mixture which must be replenished so as to maintain the required toner concentration, in the developer, typically, on the order of 4% to 7% by mass.
For high-quality printing, it is necessary to keep the mixture of toner and carrier well-controlled. If, for example, there is too little toner in the mixture, resulting images may be insufficiently dark. On the other hand, too much toner in the mixture may cause overly-dark images, or may produce unwanted contamination in the printer, as well as undesirable background on the image. As the quality demanded from printers continues to increase, it is expected that controlling the concentration of toner in the mixture will become ever more critical. This will be particularly true for obtaining high resolution, grey-level and color images.
Various techniques have been devised for measuring the toner in the mixture. One may measure the concentration of toner particles within a developer mixture or measure the developability of the developer mixture. Developability measurement actually deposits toner upon a test area so as to take into account, in addition to the toner concentration, environmental conditions, such as temperature and humidity, and other physical parameters, such as spacing, electrical bias, mass flow rate and magnetic field patterns. In each technique, any deviations from a desired state generates a signal which is used to control the addition of toner to the mixture, as necessary. Most commonly, concentration and developability sensing are accomplished by passing the developer over a surface to which the toner is attracted and optical means are used to generate the sensing signal.
Clearly, in an environment where toner powder is pervasive it would be desirable to avoid optical sensing. Therefore, it is an object of this invention to use an acoustic wave to directly measure toner concentration.